The present invention relates to methods of placing cement plugs in boreholes and more particularly, to a method for improving the integrity of such cement plugs when placed above low density drilling mud.
The methods and purposes for cementing portions of boreholes are well known. The annulus between surface casing and the borehole wall is normally filled with cement to seal off and prevent communication between aquifers. In this way, contamination of fresh water aquifers is avoided. For similar reasons, the annulus between deeper casing sections may also be cemented. In some cases, it is desirable to set a solid plug in the borehole to isolate a zone either for testing purposes or to protect lower portions of the borehole while various operations are completed above the plug. In many of these situations, the borehole below that portion being cemented is filled only with drilling mud. Quite often, that drilling mud is of a density lower than that of the cement slurry which is to be pumped into the borehole.
Drilling muds generally contain various thickening or jelling agents to increase the viscosity of the drilling mud to aid in carrying cuttings up the annulus during drilling operations. Other materials are normally added to drilling muds to increase the density of the mud to maintain downhole pressures at safe levels. If the drilling mud density is greater than that of a cement slurry which is to be placed above the mud, it can be seen that there would be little chance of loss of the cement slurry prior to hardening or of mixing between the slurry and the mud. However, in many cases, the mud density is less than that of the cement. It has normally been assumed that the increased viscosity of the drilling mud would prevent mixing between the cement slurry and the mud and would cause an interface to form which would maintain the integrity of the cement slug while it is set. However, experience has shown that in many such cementing operations, a substantial portion of the cement is lost or "falls" down the annulus or borehole through the drilling mud.